Effect

In battle

Generation V

Illusion will change the appearance of the Pokémon to that of the last Pokémon in the player's party. Illusion will not disguise the Pokémon if the last Pokémon in the player's party is fainted or is of the same species. Illusion will replicate the type of Poké Ball, nickname (or species name if none), Shininess (if any), gender, and form of the Pokémon it is masquerading as; however, it does not replicate the level of the masqueraded Pokémon. The effect is only aesthetic: it does not affect the Pokémon's stats, type, moves, etc.

The illusion wears off—causing the Pokémon with this Ability to appear as its true self—when the user is damaged directly by a damaging move or when Illusion is negated or replaced. Indirect damage or loss of HP (such as through Substitute, Life Orb, weather, status conditions, entry hazards, recoil, and so on) will not break Illusion. If the Pokémon with Illusion is hit by a multi-strike move, the illusion will be broken after the first hit.

A Pokémon with Illusion can masquerade as another Pokémon with Illusion, provided that they are different species.

Generation VI

Illusion can now mimic another Pokémon with Illusion of the same species.

Illusion now disguises the Pokémon as the last conscious Pokémon in the party.

Outside of battle

Illusion has no effect outside of battle.

Pokémon Mystery Dungeon

In Pokémon Mystery Dungeon: Gates to Infinity, the Ability functions similar to the main series games for foe Pokémon. A Pokémon with this Ability can transform into another Pokémon that can be found on the same floor of a Mystery Dungeon. For partner Pokémon, this Ability becomes nonexistent.

In the anime

Zoroark's eyes glow light blue and its outlines start to glow pink. Then, a magenta wave of energy appears by its face and moves down to its feet. As the wave moves down, its body changes its shape into the opponent, or Zoroark's eyes glow red and it causes the opponent to see illusions of its choosing.

In the manga

In the Pokémon Adventures manga

Zorua changes its appearance into something else, occasionally retaining its eyes, eyebrows, and eyelids when it does so. If Zorua takes damage, it takes a wisp-like appearance as it is forced out of its transformation.

Trivia

In Generation V, if Entrainment is used on a Pokémon with Illusion while it is disguised as another Pokémon, when its Ability activates the banner will state that the Ability belongs to the Pokémon that was used as the disguise. If the Pokémon's Ability is changed again, these messages will correctly state the Pokémon who the Ability belongs to when it activates.